A benefit with ectonurite is you can possess the others, it doesn't matter what species you are

Unless they happen to generate light, like Prypiatosian-Bs, Najlians, Pyronites and Atomix's Species.

Do they have always have that weakness? I thought it was only original/classic version that had it then later retcon it.

No, they all have aversion to sunlight without the Omnitrix-generated sun-shield. That's how Zs'Skayr always died.

Wait he dies, then how come he always comes back?

In the original series, he had Doctor Viktor reconstitute him from ashes. In subsequent ones, it's unknown. His regeneration must just be that good.